1. Field of the Invention
This invention relates to image projection systems. More specifically, this invention relates to liquid crystal light valve image projection systems.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Much progress in the state of the art of high quality large screen projectors can be attributed to the development of the liquid crystal light valve. The reflective mode liquid crystal light valve is a thin film, multilayer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer, and a photoresponsive layer. The multilayer structure is sandwiched between two transparent electrodes. A polarized projection beam is directed through the liquid crystal layer to the dielectric mirror. An input image of low intensity light, such as that generated by a cathode ray tube, is applied to the photoresponsive layer. Application of this low intensity light to the photoresponsive layer induces a spatial variation in the voltage drop across the photoresponsive layer in the pattern of the input image. The spatial variation in voltage drop across the photoresponsive layer is mirrored in reverse within the liquid crystal given the series relationship existing between the two layers. Linearly polarized projection light passing through the liquid crystal layer and reflecting from the dielectric mirrors is polarization-modulated in accordance with the input image impressed on the photoconductor and mirrored in the liquid crystal layer. Accordingly, the light valve is operative to create a replica of an image focused onto the photoconductive layer which can be projected with magnification to produce a high brightness image on a viewing screen. U.S. Pat. No. 4,019,807 issued to D. D. Boswell et al discloses such a high-performance reflective mode liquid crystal light valve.
A graphics display projector using a liquid crystal light valve of the above type is described in an article entitled "Application of the Liquid Crystal Light Valve to a Large Screen Graphics Display", published in the 1979 Society for Information Display (SID), International Symposium, Digest of Technical Papers, May 1979, pp. 22-23. More sophisticated liquid crystal light valve image projection systems are illustrated in the following patents: U.S. Patent Nos. 4,425,028, issued to R. J. Gagnon et al on Jan. 10, 1984; 4,544,237, issued to R. J. Gagnon on Oct. 1, 1985; 4,461,542 to R. J. Gagnon on Jul. 24, 1984; and 4,464,019, issued to R. J. Gagnon on Aug. 7, 1984.
In many conventional liquid crystal light valve image projection systems, three primary color images (red, green and blue) are displayed in optical registration on a viewing screen. The three primary color images are typically generated by three separate light valves, which are projected on the screen by three separate projection lenses. The projection lenses are arranged about an optical axis which passes through the composite image displayed on the screen. In such an arrangement of projection lenses it is necessary that the three primary color images converge in the plane occupied by the screen. In order to effect such convergence, complicated and expensive optical arrangements with accompanying electronics are typically required.
Moreover, in systems incorporating multiple light valves each is typically addressed by a separate laser beam. The addressing (writing) laser beams are amplitude modulated in response to a video signal corresponding to either a red, green or blue primary color image. A separate beam deflection network is used to scan each beam in the horizontal and vertical directions over the writing surface of the light valve. In this way the red, green and blue primary color images encoded in an input video signal are replicated in the three light valves.
Such multiple light valve displays allow for high resolution image projection, but are expensive to implement. As noted above, a separate liquid crystal light valve is required to engender each of the primary color images and a separate beam deflection network must necessarily be employed in association with each light valve. Accordingly, a full-color image display system utilizing only one light valve and a single associated beam deflection network would be significantly less costly than existing display systems. In addition, such a single light valve system could be configured to occupy substantially less volume than those incorporating three light valves.
Hence, a need in the art exists for a full-color image display system having only a single light valve.